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I have been running my diy cnc mill spinde at full speed (3500 rpm ish) and it is getting excessively hot (ie too hot to touch for more than 5 seconds). The bearings are cheap taper rollers with a 1 " bore and I have tried oiling them with light oil and standard wheel bearing grease but are as hot in both cases. I did cheap out on the taper rollers as they are for trailer wheels and so I am not expecting super precision, however looking at similiar size cheepo TR bearings on the net the limit is 7800rpm with grease. They have a rubber seal on the outboard bearing and I have used this to seal the lower one. The top one is sealed using a close running 'labyrinth' but both are getting too hot. I have tried various preloads including as close to none as I could get but it does not seem to make any difference.

Does this sound like a bearing problem, bearing misalignment problem or lubrication issue? I am not aiming for super precision but usable would be nice! The spindle in the workshop series of books 'Spinles' book uses similiar automotive bearings.

Andrew,
My take on your overheating problems is “yes” to all the above..

First off, I assume you like me did NOT do an initial run in procedure to “break in” the bearings. Information I found when I had to replace the bearings on my mill/drill was:

” Typical run in for spindles is a starting point of about 400 rpm,
up to max in 500 rpm increments, in steps of 30 - 45 minutes.
Check housings for excessive heat as you go.
Then reverse and do it over!
Watch temperature. If it gets hot, stop let cool and do over.

The Fafnir Superprecision Machine Tool Bearing catalog suggests that with optimal design,
bearings running synthetic low friction grease ought to be able to run about 20 degrees C
over ambient or about 40 degrees F. In fact, they suggest a proper break-in procedure temperature of 130 degrees F.
involves running the spindle until it reaches 150 degrees F, turning it off to cool, and repeating the cycle
until the spindle stabilizes at 130 degrees F or lower operating temp.
The ideal equilibrium operating temperature is 95 to 115 degrees F. Maximum temperatures should not exceed
170 degrees F. Timken's published specs for tapered roller bearings, including bearing life specs, assume an operating
temperature of 130 degrees F.”

To this end, I got one of those cheap non contact thermometers off e-bay and proceeded to make runs at various speeds and when the temp got up, stopped and waited for it to cool off and repeat. I was surprised as to how the max temps dropped quite quickly. As I say, I had never done a proper breaking on my machine and up until the day the bearings tore up, I had high temp problems. When I increased the max spindle speed to 4000 rpm, it got worse.
Next thing, is to use a proper grease NOT wheel bearing grease as it is to thick. A lot of people recommend Kluber 15 and only fill to about ~30% as I remember. To much grease will also cause more heating problems.

Preload can also be a problem and trying to get it set up properly can also be a pain.
When I did my quill, I set it up in the lathe so I could easily adjust the preload before putting the quill back into the mill. This also gave me several additional rpms to ramp up the speeds. The other thing I did, was to setup a spreadsheet and recorded all of the various speeds, temps, etc. The good news is having 4000 rpm at a max of 135° F (~57°C)

As to the problem of seals, my machine has no direct contact seals on either the top or the bottom bearings. I feel sure that at these kind of rpm, this could also cause a problem.

I used a pair of the cheapest taper roller bearings on eBay in my 4th axis/CNC lathe spindle that I posted about a few weeks ago. They're slightly bigger bearings and didn't get hot at all running at 3000rpm which was the fastest I ran it. The bearings were greased, however oil is better to reduce friction and therefore heat.

It might be that you simply got a particularly bad set of bearings. I've bought several of the size bearings you linked to on eBay for various projects and found that with some you could tell without mounting them that they felt less smooth. If I lay the bearings flat and give the outer race a spin the time is stays spinning for varies a lot. The SKF bearings I got cheap on eBay were definitely better.

Because you mentioned I just went to my workshop to check my spindles taper bearings http://malamuterun.co.uk/homemade_cnc_mill_spindle.htm
It was running for good 4 hours cutting a metal and spindle around bearing is hot. I did not break them in at all. Just wonder, are they heating up because they are submerged in oil (run @ 3000rpm) or because I didn't follow breaking procedure ??

Waitroda,
How hot do your bearings get? I did not measure mine but I estimate over 60-70 deg C as I could not keep my fingers on the housing. Much higher and I think that the grease would start to smoke!

I have a plan of action on my spindle:

Bin the cheepo bearings and go for some middle of the road koyo bearings (still not expensive at 10 quid each). I could have gone with Timkens at twice the price but I think these should suffice.

Replace the rubber sealed lower with an aluminium labyrinth with a couple of thou clearance over the shaft.

Grease with short fibre grease (I cant afford the kluber grease and I think a decent bearing grease should suffice at the speeds that I am looking at)

Run it in a more managed way.

Possibly look at increasing the speed of the spindle up to 4500rpm in future if 3500 proves successful

It might be worth pulling the seals off the existing bearings to see how much difference it makes? If there the same as the seals that were on my bearings then they do come off with some 'encouragement' and it does appear to make a big difference to the coefficient of rolling resistance. I removed the seals from these bearings in my wind turbine for this reason and covered up the bearing with a piece of steel machined to be a close fit.

Waitroda,
How hot do your bearings get?
I will update my build log to show the progress.

I was running today machine non stop for 4 hours and after I couldn't keep fingers on spindle as well. I never paid too much attention to bearings temp until I read your post :) That's the DIY spindle filled nearly entirety with synthetic engine oil with a little air gap. When I mill next time I'll do the temp checks in some time periods.
I'm awaiting for your update

I hope I've understood you problem I I think it might be the seals ( as you suspect) and the oil!

firstly wheel bearing seals are designed to keep al sorts of nasties out but at relativly low speeds, once you put them in a spindle the extra friction is a problem, low or zero friction seals are a must in a spindle.

Secondly over greasing or over oiling is the worst thing you can do to a spindle with regards to heat, as the shearing action within the oil that allows the parts to move easily generates heat.

as Bill mentioned it is easy to over tighten the bearings, but it depends on the orientation of the bearings. if they are face to face then if I remember correctly you need to preload them so that as the shaft grows under heat the correct loaing is acheived as the extra length reduces the loading, BUT if they are back to back then any heat generated will add extra load on the bearings and in turn generate more heat.......visious circle ect.

This could be be your problem in that the original seals are generating a bit of heat(plus to much oil/grease) which in turn is putting load on the bearings, which is then generating heat and so on and so on.....

Im no expert but I have been looking in to this for my own spindle and it seems that spindles are suseptible to any small change,be it heat, vibration to over loading the tool.

sorry for the ramble, just ignore it i have got your problem it all wrong. Let me know how you get on tho

Thanks all for the advice. I am still waiting for the new bearings so I have not made much progress on the problem yet. I have made a guard for the spidle belts and am making way covers. But when I get around to it I will post on my build thread.